This invention relates to polyolefin compositions having improved properties and to a method of preparing such compositions. More particularly, this invention relates to a process for improving the properties of graft-modified polyolefins.
The preparation of graft-modified resins by grafting unsaturated polycarboxylic acids, esters or anhydrides such as maleic acid, maleic anhydride, or a maleic acid ester to an olefin polymer or copolymer to improve the adhesive properties of the resin is known. Such graft-modified polyethylene, polypropylene and copolymers thereof have been described in the literature and are available commercially.
The modification of polypropylene polymers, both crystalline and amorphous by reaction of maleic anhydride has been described in Belgian Pat. No. 652,324. In U.S. Pat. No. 3,414,551, an improved process for reacting maleic anhydride with crystalline propylene polymer is described. In general, from about 0.2 to about 10% of maleic anhydride is incorporated into the polymer utilizing an organic peroxygen compound in the absence of any solvent. British Pat. No. 1,441,189 describes a process for preparing a graft-modified ethylene polymer or copolymer by reacting an ethylene polymer or copolymer with a dicarboxylic acid graft-copolymerizable therewith or with a graft-copolymerizable derivative of such a dicarboxylic acid in an alkyl-substituted aromatic hydrocarbon solvent in the presence of a free radical initiator. The dicarboxylic acid or its derivative is added gradually to a mixture of the solvent and the starting ethylene polymer or copolymer during the grafting reaction.
Additional examples of patents and publications which describe the preparation of graft-modified polyolefins include U.S. Pat. Nos. 3,856,889; 3,480,580 and 3,481,910, Japanese Patent Publication No. 15422/69 published on July 9, 1969 and Japanese Patent Publication No. 6384/64, published on May 4, 1964. U.S. Pat. No. 3,856,889 describes blends containing polyethylene and graft-modified polyolefins. These blends are reputed to have good melt stability and good adhesion to paperboards and inks. Stabilizers can be included in the blends to prevent gelation or degradation of the blend properties, and these include dilauryl thiodipropionate, butylated hydroxytoluene, dioctadecyl p-cresol and 2,2'-methylene bis(6-tert-butyl-p-cresol).
U.S. Pat. No. 4,146,590 discloses a process for the modification of polyolefins by the combination of such polyolefins with one or more alicyclic carboxylic acids having a cis form non-conjugated double bond in the ring, alpha,beta-unsaturated carboxylic acids, their anhydrides or mixtures thereof, in the presence of a radical producing agent in an extruder, wherein during or after the above described addition reaction, the modified polyolefin under molten state is reacted with one or more polyfunctional compounds having at least two alcoholic hydroxyl groups or amino groups. This patent teaches that the level of addition of the polyfunctional compound is in the range of 0.1 to 2 molar equivalents or less, preferably 0.1 to 1.2, to all the carboxylic acid or anhydride thereof contained in the modified polyolefin. The working examples provided in this patent teach the utilization of molar ratios of the polyfunctional compounds to unreacted carboxylic acid or anhydride thereof of less than 2:1.